Hot deserts

where are hot deserts located?

roughly 30 degrees north and south of the equator

tropics if cancer and Capricorn

area of high pressure with dry air, little rainfall or clouds

How many mm of rainfall does a desert have (arid)?

less than 250mm of rainfall per year

how many mm of rainfall does a semi-desert have a year?(semi-arid)

250-500mm per year

what are the three types of desert landscape?

• Hamada - bare rocky surface eg plateux

• reg - stony desert with rock fragments scattered over a large area

• erg - predominantly sand (‘sand sea’)

is a desert an open or closed system?

open

what are the inputs of a desert system?

* precipitation
* solar radiation
* descending air in the ITCZ

what are the outputs and stores of a desert system?

outputs = water, sediment and reradiation from earth’s surface into atmosphere

stores = landforms, vegetation and soils

what are the processes/flows in a desert system?

isolation, oxidisation, wind action, wind erosion, transport, overgrazing, water management

what are the feedback loops in a desert system?

• positive = absence of vegetation can affect the climate as less evapotranspiration, this leads to drier climate with less rainfall, lead to less vegetation

• dynamic equilibrium = seasonal cycles of wind can lead to small scale and short term adjustments in sand dune profiles, but year to year their shape stays relatively the same

what atmospheric processes are related to the distribution and aridity of hot deserts?

global atmospheric circulation

air rising in the intertropical convergence zone sdreads north and south at the tropical hadley cells

due to the sinking between the hadley and ferrel cells air become drier and warmer creating a belt of high pressure (30 degrees north and south)

causes cloudless skies and lack of rainfall. therefor there is extreme temperatures - aridity

what is aridity

severely dry climate

briefly explain the importance of latent heat in the desert?

• water evaporated and changes from liquid to gas using latent heat from the environment, causing a drop in temp

• opposite occurres during condensation when heat is expended

• in dry climates evaporational cooling is minimum in daytime (no moisture) so there are very high temps

• at night there is little water vapour to trap longwave radiation so there is rapid cooling

How does continentality effect aridity?

• aridity is higher in continental interiors eg central africa asia and australia.

• due to reduced influence from moist airstreams from oceans

• continental exterior deserts tend to be semi-arid as occasional storms can bring rain

How does relief effect aridity?

prevailing moist ocean winds can be forced up mountains

this means it is forced to rise and cools creating precipitation that mostly falls on the windward slopes

after precipitation the cool air sinks back down the sheltered side of the land drying and warming - creates a band of high pressure

this is called a rain shadow

How do cold ocean currents affect aridity?

some deserts are on coastlines of cold ocean currents eg (Atacama desert)

air that is in contact with the ocean currents is cooled and being denser and cooler than the warm air, remains close to the ground

the air passing over the oceans onto the land is also cool and unable to pick up moisture meaning the land remains very dry with little ability to form precipitation

what are some distinctive aspects of the water cycle in hot deserts?

• rainfall low and sporadia (mostly occurs in heavy downpours causing overland flow and high evaporation)

• actual evapotranspiration is low however rates of potential evapotranspiration are very high (up to 10x annual precip)

• rivers have distinctive sources: exogenous, endoreic and ephemeral

What is the aridity index?

most common measure of aridity

ratio between annual average precipitation (P) and average annual potential evapotranspiration (PET)

arid areas have a ration of less that 0.2 = rainfall supplies less that 20% of water needed for optimum plant growth

semi arid = 0.2-0.5

What is the hot desert water balance?

relationship between precip, soil moist storage, evapostans and runoff

precipitation = evaporation ± soil water storage + runoff

What are the three main energy sources in hot deserts?

• insolation - the amount of heat (shortwave radiation) that reaches the earth’s surface - this is high in deserts leads to

  • intense weathering due to extreme temperatures

  • high rates of PET

  • variable warming creating localised winds

• water - following infrequent storm can flow in a sheet or in wadis, erosion very significant in a short amount of time.

• wind - driven by the atmospheric circulation system. 20-25% of desert surfaces made from sand/small sediment that can be transported by wind.

What are some sources of sediment in hot deserts?

• mass movement (rockfalls)

• sand/finer sediment carried into desert by winds

• erosion of rocks

• weathering

What are sediment cells and their componenets?

the movement of sediment, commonly applied to coastal environments, that has….

• inputs - sediment

• outputs - sediment transported away from desert (wind/water)

• sediment sinks - areas of deposition

• transfers - movement of sediment (wind/water)

What is the sediment budget?

considers relative amount of sediment in each part of hot desert system.

What are the two main types of weathering in hot deserts?

• mechanical (physical) = gradual disintegration of rocks without any chemical change

• chemical = change in rocks due to chemical action, usually in the presence of water

What is the most common mechanical weathering in hot deserts?

thermal fracture (insolation weathering)

• caused by intense temp fluctuations, where rock expands when heated and contracts when cooled

• believed to be due to the presence of moisture eg dew

• dependent on colour, lithology (rock type) and geological structure

less common include…

salt crystallization due to evaporation of salts dissolved in soil forms layers on surface that can break rocks

frost shattering, due to temp below freezing, water freezing in cracks and expanding causing breakage

What are the 4 types of thermal weathering?

• block separation - if rocks have clear pattern of joints whole blocks will break off

• shattering - rocks without joints or fragments to shatter of into angular fragments

• exfoliation - repeated heating and cooling on surface of rock causes peeling or flaking

• granular disintegration - crumbling or break down into individual grains

what is chemical weathering

• due to chemical action rocks change in the presence of water

• rocks containing salts are vulnerable to being dissolved (solution) leading to crystal growth which can damage and corrode buildings

What are the two main types of wind erosion in hot deserts?

• deflation = removal of loose material from desert floor, resulting in exposed underlying rock. over time leaving behind only larger sediment

• abrasion = ‘sandpaper effect’. wind is blown against rock surfaces, carving or sculpting them into various shapes, mainly occurring within m of the desert floof

What are the 3 types of wind transportation?

• surface creep = particles rolled along the floor

• saltation = sand particles move in series of leaps as they are picked up and dropped by gusts of wind

• suspension = fine sands and clays picked up and carries for considerable distances, potentially out of the deserts, common in high velocity winds

How does wind deposition operate in the desert?

• occurs when wind velocity falls below critical value where it can carry and move sediment

• common that sediment will be deposited in sheltered areas from the wind like the leeward side of a rock formation

what are the various aeolian landforms?

• deflation hollows

• desert pavements

• ventifacts

• yardangs

• zeugan

• sand dunes

What is a deflation hollow?

• depression created by strong winds that erode loose material from the desert surface.

• depth is controlled by several factors such as wind dynamics, depth of loose material above consolidated and water table

• saturated sand in the hollows can lead to the development of salt lakes etc

What are desert pavements?

or regs

• suface covered with rock fragments resembling a cobbles pavement.

• finer sand is blow away leaving this exposed

• common on well developed aluvial fans due to deflation

What are ventifacts?

small scale featured found on desert flood

rock fragments the size of pebbles with an eroded facet (face) aligned with the prevailing wind.

some have a single facet - one wind direction

others have multiple indicating changes in wind direction (multidirectional)

What are yardangs?

• elongated ridges separated by deep groves cut into the desert surface

• vary in size from cm- 100s of m and km in height

• weaker rocks eroded by abrasion to form troughs in the direction of prevailing wind

• tougher rocks left upstanding

• don’t develop in regions with multidirectional winds

What are zuegen?

similar to yardangs up to 30m high

however develop in horizontally layered rocks

gives pedestal shape with a flat capped rock

primarily due to abrasion which occurs only a couple meters above floor

means top is often larger

what are 2 common types of sand dune and how to they form?

barchans

• a dune develops due to sand deposition, with sand being blown to a crest and then down a slip face which is steeper than the prevailing side

• more sand blows around the base of the dune creating a crescent shape (the horns of the dune are extended around the base)

seif dunes

• a barchan can develop into one when the wind direction changes and one horn is extended more than another

• commonly found in extensive areas of sand (sand seas)

• other slip faces may develop as it elongates

How do sand dunes tend to start?

some deposition of sand on the side of a rock. this increases over time depositing more and more sand and being shaped by the wind.

what is desertification?

‘destruction of biological potential of the land resulting in desert-like conditions’ - UN

• destruction of ecosystems and habitats

• unproductive wasteland

• erosion to soil and death of vegetation

where are the most vulnerable locations for desertification?

margins of hot deserts

what proportion of the world land surface is being affected by desertification?

approx one third

what are the natural causes of desertification?

CLIMATE CHANGE

natural climate change causes a fluctuation in global climate across the world

8000 years ago north Africa and middle east were much wetter . evidence; fossil aquifers, fossil plant remains.

human induced climate change has increased global temp by 1 degree census since 1880 (greenhouse effect)

What are the human causes for desertification?

• over-cultivation = intense farming on land that lacks organic matter and is fragile can reduced soil fertility and cause erosion

• overgrazing = if an area has too much livestock living on it, vegetation and soil will suffer as carrying capacity is exceeded

• over-irrigation = if plant sare overirrigated causes salts in soil to dissolve and forms a crust on surface when water evaporates (infertile impermeable soil)

• population increase = Sahel region doubles every 20 years (population 260 million) meaning land needs to be more productive

• firewood = semi-arid regions may rely on firewood used for cooking, unsustainable and exposes soils to erosion

• tourism = safaris cause damage to vegetation and soil

What are the impacts of desertification on ecosystems?

• loss of nutrients due to over-exploitation by agriculture, reducing biodiversity

• loss of top soil which is the most nutritious part of the soil due to exposure to wind and water

• increases salinity with irrigation

• depleted water sources affecting water table

• vegetation destroyed impacting animals and meaning they must migrate

why is the rate of desertification an issue?

desertification is a relatively rapid process but the development of vegetation, soils, animals (the development of an ecosystem) can take hundreds of years to establish.

What are the impacts of desertification on people?

• lack of water = disease spread and hygiene issues

• loss of vegetation = less produce, meaning people abandon settlements

• dust clouds affect air quality and effect health for even people far from desert regions

• food insecurity (25% of India’s land turning into desert) compromising ability to feed people

• depleted grassland means people cannot sustain animals so people migrate to overcrowded cities.

what are the impacts of desertification on landscapes?

• sand dunes are created as material is dry and no longer trapped by vegitation

• soil erosion can cause deep gullies to form in the landscape

• sandstorms become more common when protective vegetation is removed - increased abrasion

• landslides due to loss of vegetation

What is the climate of a hot desert like?

• many hours of sunshine due to no cloud cover (zones of high pressure)

• temperature variations of 30 degrees c day to night due to lack of cloud cover

• sandstorms especially in areas with large variation in temp

what are the characteristics of desert soil?

• too dry and sandy to support much vegetation so soil is poorly developed (no hummus due to lack of water)

• red color due to oxidation with iron rich rocks

• very porous and permeable with little water retention however leaching is not a problem so with correct management crops can be grown

What are the characteristics of desert vegetation?

there are some plants that adapt (xerophytes) due to the hostile climates that don’t support growth…

• drought avoidance = plants live for only one season, dry up and die in drought season but store the necessary nutrients. some also use taproots

• drought resistant = plants adjust their metabolism to be able to become dormant

• water storage = succulent plants have shallow roots to quickly absorb water in storm events, stored in fleshy leaves, roots and stems

What is an oasis?

dense pocket of vegetation created by water existing close to the surface

eg water may be trapped due to impermeable layer near the surface resulting in lush vegetation forming important habitats

How is climate change impacting desertification?

by middle of this century world’s dryland may increase in temp by 2-5ºC, accelerating rates of desertification

• increased rates of ET

• soil water decrease - settlement and agriculture less viable (people migrate) causing a positive feedback loop of desertification then occuring in other areas

• droughts more frequent and severe

• rainfall decreases as the dryland expands

How can planting trees help desertification?

• roots hold soil together reducing risk of erosion

• ground gains nutrients due to leaf litter

• shade for the soil reducing evaporation and reducing temperatures

• great green wall sahel

how can soil management help with desertification?

* allowing land to rest between crops an grazing allows nutrients to recover
* crop rotation means nutrients are not stripped from the soil
* local people can make their own compost to help nourish the soil

How does appropriate technology help with desertification?

* using cheep and sustainable materials that locals can access and use
* for example stone lines across land that is level can trap soil being transported by the wind so it is not eroded
* This also prevents overland flow that strips the nutrients in the soil

How have people in the Badia, Jordan addressed desertification?

desertification in prevalent from overgrazing creating unproductive land.

2002 the tal rimha rangeland rehabilitation project make agreements with farmers to revere this. stone walls were constructed to control and retain water

water was diverted down ditches where drought tolerant shrubs have been planted; they grow quickly, provide firewood, hold soil together and encourage biodiversity.

How successful was the 2002 tal rimah rangeland rehabilitation project?

2004-2008 plant and animal species more than doubled

birds nest in the base of shrubs

animals are returning to the area

sustainable living is now benefiting locals, the land and the economy

what are the 4 main sources of water in Hot deserts?

• exogenous rivers = have their source in mountains outside deserts, have sufficient water flow, eg the river Nile despite high evap

• endoreic rivers = rivers that flow into deserts but terminate at inland lakes or seas, eg the dead sea

• ephemeral rivers = rivers that flow intermittently in deserts, flow after storm events or snow melt. flow rates vary largely and can be strong sources if erosion.

• episodic flash floods = infrequent rainfall events that involve torrential convectional storms, lost of water in short period, sunbaked soil means there is lots of overland flow (erosion). sheet flooding or channel flooding.

what are the water processes of erosion in deserts?

• hydraulic action - the force of water

• abrasion - sandpaper effect of loose rocks

• corrasion - fragments of rock carried by water gouging bedrock

• solution - dissolved sediment

what are the 4 river processes of transportation

• traction = rolling sediment along the channel bed

• saltation = bouncing particles

• suspension = sediment carried in the body of water

• solution = dissolved sediment

what are the 5 sheet wash processes in deserts?

• splash erosion - force of falling rain water displacing soil

• sheet erosion - water running over a large impermeable surface as overland flow washing particles away

• rill erosion - sheet wears away soil to form defined paths (rills)

• gully erosion - over time rills become deeper and wider to form gullies

• bank erosion - fast water flow wears away channel sides and causes widening

explain the formation of a wadi…

* dry riverbed or valley
* vary in size and features
* steep edges due to sever erosion in periods of high water flow
* flat bottoms infilled with sediment - boulders, pebbles as finer material is removed by the wind
* there are often many channels that split up and rejoin due to flashy water flow

explain the formation of an alluvial fan

* sediment is washed out of mountainous areas and disperses onto the leveling out land.
* as the water spreads from the mountain the larger sediment is deposited due to the reducing energy. this gives the appearance of layering of sized of sediment.
* they can extend several kilometers away from the mountain edge with a thickness of up to 300m.

what is a bajada?

* when numerous alluvial fans merge to created a continuous layer of sediment across the mountain front
* they also exhibit clear sorting of sediment
* playas may form on lower bajadas where they become extensive flat deposits

What is a pediment?

gently sloping rock surface at the base of a desert mountain range

low angled and may be completely. covered in sediment or bare

what is a playa?

salt lakes

how do playas form

* water flows over the alluvial fans and bajadas carrying fine material onto the desert floor
* some of this water will percolate through course sediment and travel as throughflow to a depression where it may form a playa
* this is an enclosed desert lake with no outflow. large surface area will encourage evaporation and a salty crust will often form around the edges
* in some places this salt can be collected and used commercially

what is a canyon?

steep sided gorges associated with rapid down cutting into plateaus

How do canyons form?

* rapid down cutting in a plateau
* can be formed by exogenous rivers that run consistently over a spot
* grand canyon Arizona due to the Colorado river showing ancient layers of rock dating back up to 2 billion years ago.

what is a mesa

Mesas are table-like raised plateaus with steep sides bordered by wadis or canyons

what is a butte

Buttes are pillar-like land forms that are the result of the erosion of mesas. Typically they have scree around their base due to weathering and mass movement.

what are inselbergs?

develop in rocks like granite and sandstone with absence of layering. they are more rounded than mesas and buttes. some scientists believe they formed due to chemical weathering in the past when humidity would have been higher.

how have scientists investigated historical extent and distribution of deserts?

carbon dating methods and pollen analysis to study patterns of vegetation

How has the extent and distribution of deserts changed over time?

• has remained unchanged for past 5000 years due to relatively stable climate with GAC as the main driving force

• 8000 years ago - interglacial period with humid warm conditions with widespread forests, low aridity

• 20,000 years ago - glacial maximum aridity was widespread. deserts in far north were cold and in the south were distributed similar to today but far more extensive